structure Ndc = 
struct
  (* ndc = normalised device coordinates *)

  fun ltrbToVertex (left, top, right, bottom) =
    #[ left, bottom
     , right, bottom
     , left, top

     , left, top
     , right, bottom
     , right, top
     ]

  fun ltrbToVertexRgb (left, top, right, bottom, r, g, b) =
    #[ left, bottom, r, g, b
     , right, bottom, r, g, b
     , left, top, r, g, b

     , left, top, r, g, b
     , right, bottom, r, g, b
     , right, top, r, g, b
     ]

  fun fromPixelX (xpos, windowWidth, windowHeight) =
    let
      val halfWidth = Real32.fromInt windowWidth / 2.0
      val xpos = xpos - halfWidth
    in
      xpos / halfWidth
    end

  fun fromPixelY (ypos, windowWidth, windowHeight) =
    let
      val halfHeight = Real32.fromInt windowHeight / 2.0
      val ypos = ~(ypos - halfHeight)
    in
      ypos / halfHeight
    end
end